The present invention relates to radar mapping; and more particularly to an improved method and system for real aperture radar ground mapping.
A conventional real aperture mapping radar is typically a non-coherent system that utilizes the return power or energy seen by the radar antenna to make a paint of the ground as the antenna scans the area being mapped. The resolution in the range dimension is determined by sampling the reflected radio frequency energy at a selected rate, thus creating range gates. However, the angular or azimuth resolution is determined by the angular width (beamwidth) of the transmitted beam of RF energy. Azimuth resolution is relatively poor when compared to reasonably achievable range resolution. For example, the range resolution for a map at ten nautical miles may be 150 feet; but the azimuth resolution for a beam width of three degrees is about three thousand one hundred eighty feet at ten nautical miles. Thus, the location of a corner reflector or other target with respect to range can be determined to be anywhere within a one hundred fifty foot range interval, but the location in azimuth of the reflector can be anywhere within the three thousand one hundred eighty foot azimuth interval.
Range resolution can be improved readily by increasing the sampling rate. However, improvement in azimuth resolution of a real aperture ground mapping system was heretofore achieved only by decreasing the width of the transmitted beam. This could be accomplished by either increasing the physical size of the radar antenna, or by increasing the frequency of the transmitted RF energy. Neither of these solutions were attractive. Increasing the physical size of the radar antenna is costly at best; and because the ground mapping radar is mounted in an airborne platform, it may not readily support the extra weight, or there may be insufficient space to accommodate the larger size antenna. Moving to a higher frequency also has its disadvantages. For example, at higher frequencies there is a greater attenuation of the reflected energy in the presence of rain; and because of considerations related to other modes of the radar system, it may not be possible to use a frequency that gives a desirably small beamwidth.